In today's 3rd- and 4th-generation wireless communication systems, complex modulation schemes are used to improve the spectral efficiency of the signals and thus increase system capacity and throughput. In contrast to the constant envelope signals used in earlier wireless systems, the signals in these high-speed wireless networks typically have high peak-to-average power ratios. As a result, the power amplifiers used in system transmitters must be operated in modes that are more linear, but thus less efficient. To increase the efficiencies of the power amplifier system, which in turn reduces system temperatures and power consumption, the well-known Doherty power amplifier (PA) circuit is commonly used. Often a feed-forward or feed-back system allows the amplifier to operate closer to the saturation region for achieving higher efficiency.
FIG. 1 illustrates a diagram of a conventional Doherty PA circuit 100. The Doherty PA circuit 100 has two amplifiers circuits in parallel: a main amplifier circuit 10 comprising a main amplifier 102 and a peak amplifier circuit 20 comprising a peak amplifier 112. Amplifier circuits 10 and 20 each amplify an input signal provided to the two amplifier circuits via an input signal splitting network (not shown). The outputs from amplifier circuits 10 and 20 are combined at a combining node 120 and provided to a combining network 30 which outputs the amplified signal to an external load 140. Amplifier 102 is called ‘main amplifier’ (or simply ‘main’), because it provides amplification across all instantaneous output powers. Amplifier 112 is called ‘peak amplifier’ (or simply ‘peak’) as it only contributes power only after the main amplifier 102 saturates. The operating mode in which only the main amplifier 102 supplies the desired output power and the peak amplifier 112 is deactivated (or in an off state) is called a back-off mode (or back-off state or back-off regime). When both amplifiers are activated (or ‘on’), the operating mode is called a full-power mode. Amplifiers 102 and 112 are each presented at their output with an impedance that is optimal for their respective rated output power.
In order to match impedances for achieving maximum power transfer from the amplifiers 102 and 112, matching networks 106 and 114 are required. An impedance inverter 110 is used within the main circuit 10 to address the load modulation caused by the peak amplifier 112. Within the combining network 30, a transmission line 118 is used to transform the intermediate impedance at combining node 120 to the overall impedance of the amplifier circuit 100 required by the system. Occasionally, a short transmission line 116 is required to transform the impedance of peak amplifier circuit 20 to a high impedance, when amplifier 112 is in the off state. This prevents RF (radio-frequency) power from the main circuit 10 from leaking to the peak circuit 20 and from detuning the optimal load impedance seen by the main amplifier circuit 10.
FIG. 2 illustrates general efficiency and current curves with respect to relative powers for the amplifier circuits 10, 20 and the overall Doherty PA circuit 100. The Doherty power amplifier circuit 100 achieves overall high efficiency by turning on the peak amplifier 112 whenever the envelope of the input signal peaks. The current generated from the load of the peak amplifier 112 modulates the output load of the main amplifier 102, keeping the voltage swing high and keeping the main 102 operating in the high-efficiency region. When the Doherty PA circuit 100 operates at full power, main 102 is designed to see optimal impedance. When the Doherty PA circuit 100 operates at back-off power, the main amplifier 102 sees an impedance of twice the load impedance. The variation in load impedance associated with full power and back-off power operation improves the overall efficiency of the Doherty PA circuit 100.
For the Doherty power amplifier circuit 100 to be operating at the highest efficiency at back-off power, it relies on the peak amplifier circuit 20 to be completely off and isolated from the main amplifier circuit 10 at the combining node 120. This is achieved by designing the peak amplifier circuit 10 such that its impedance, when observed from the combining node 120, is ideally open such that no RF power from the main amplifier circuit 10 can leak through the peak circuit 20. However, with the configuration in FIG. 1, such design is only achievable for a narrow band of frequencies. This prevents the Doherty PA circuit of FIG. 1 from operating over wide bandwidths and from being used for multiband/dual band applications with high efficiency and linearity.